(4bo) Plastic Electronics: Structure-Property Relationships of Polymer Semiconductors in Thin Film Transistors

The era of plastic and organic electronics has emerged in the past decade towards the development and applications of low cost electronics and energy conversion systems.^1,2 Understanding of the charge transport properties of polymer semiconductors is critical to the future advances in plastic electronics. Our  research aimed to understand the relationships between molecular structure, processing, morphology, and charge transport properties of polymer semiconductors. Organic field effect transistors (OFETs) are excellent platforms to study the charge transport properties of diverse conjugated polymers. The polymer thins were also studied by UV/Vis absorption and photoluminescence spectroscopies, cyclic voltammetry, X-ray diffraction, optical microscopy, atomic force microscopy, scanning and transmission electron microscopies. In addition, various molecular and interfacial engineering approaches were pursued to understand the physical factors that influence the performance of polymer transistors and to develop high-performance devices.

Our studies of structure-processing-morphology-property relationships in n-channel,^3,4 p-channel,^5,6 and ambipolar^2,7,8 polymer transistors revealed that variations in molecular structure and processing can be used to tune electronic energy levels, solid-state morphology and crystallinity, and thus charge-carrier mobility of the polymer semiconductors. The studies resulted in achievement of high-performance transistors with high mobility (1.5 cm²/Vs).⁵ We found that the long-term air-stability of n-channel and p-channel polymer transistors depended on the crystallinity (morphology) and electronic energy levels of the polymer thin films.^3,6

Interface engineering was applied to reveal the effect of dielectric constant of a gate insulator layer on charge transport of n-channel polymer transistors.⁹ The polymer transistors with a thin polymer dielectric buffer layer exhibited excellent charge transport properties, such as higher mobility, negligible hysteresis between forward and backward scans, and superior electrical stability under multiple cycles of gate voltage scans, compared to the device without the buffer layer. Electron mobility was found to increase exponentially with decreasing dielectric constant of the polymer dielectric buffer layer. These findings highlight the importance of engineering the dielectric/semiconductor interface for a better understanding of electron transport in polymer semiconductors and for developing high-performance OFETs.

We have also studied polymer semiconductor nanowires and their nanocomposites with an insulating polymer matrix as a means of controlling the solid-state morphology of active thin films in transistors.¹⁰ Well-dispersed polymer semiconductor nanowires with a high aspect ratio in an insulating polymer matrix resulted in high mobility (~0.02 cm²/Vs) and DC conductivity (7 mS/cm) with a low percolation threshold (0.5 wt%). Intra- and inter-nanowire charge transport was proposed and examined as the origin of the observed high mobility/conductivity and composition dependence.

Selected Publications (30 total; total citation >1000).

(1) Kim, F. S.; Ren, G.; Jenekhe, S. A., Chem. Mater. 2011, 23 (3), 682-732.

(2) Kim, F. S.; Guo, X.; Watson, M. D.; Jenekhe, S. A., Adv. Mater. 2010, 22 (4), 478-482.

(3) Briseno, A. L.; Kim, F. S.; Babel, A.; Xia, Y.; Jenekhe, S. A., J. Mater. Chem. 2011, 21 (41), 16461-16466.

(4) Guo, X.; Kim, F. S.; Seger, M. J.; Jenekhe, S. A.; Watson, M. D., Chem. Mater. 2012, 24 (8), 1434-1442.

(5) Subramaniyan, S.; Kim, F. S.; Ren, G.; Li, H.; Jenekhe, S. A., Macromolecules 2012, 45 (22), 9029-9037.

(6) Ahmed, E.; Subramaniyan, S.; Kim, F. S.; Xin, H.; Jenekhe, S. A., Macromolecules 2011, 44 (18), 7207-7219.

(7) Kim, F. S.; Ahmed, E.; Subramaniyan, S.; Jenekhe, S. A., ACS Appl. Mater. Interfaces 2010, 2 (11), 2974-2977.

(8) Wu, P.-T.; Kim, F. S.; Jenekhe, S. A., Chem. Mater. 2011, 23 (20), 4618-4624.

(9) Kim, F. S.; Hwang, D.-K.; Kippelen, B.; Jenekhe, S. A., Appl. Phys. Lett. 2011, 99 (17), 173303.

(10) Kim, F. S.; Jenekhe, S. A., Macromolecules 2012, 45 (18), 7514-7519.